In organic light-emitting diodes (OLEDs) only a portion of the light generated is outcoupled directly. The remaining light produced in the active region is distributed to different loss channels, thus in light, for instance, which is carried in the substrate, in a transparent electrode and in organic layers due to waveguide effects, and in surface plasmons, which can be produced in a metal electrode. The waveguide effects occur in particular due to the differences in the refractive index at the boundaries between the individual layers and regions of an OLED. In known OLEDs only roughly a quarter of the light generated in the active region is typically outcoupled into the environment, thus air, for example, while roughly 25% of the light produced is lost for radiation due to wave guidance in the substrate, roughly 20% of the light produced is lost due to wave guidance in a transparent electrode and the organic layers and roughly 30% is lost due to the generation of surface plasmons in a metal electrode.
Furthermore, the effect of the aforementioned loss mechanisms differs depending on the spectral proportion of the radiated light that is considered. The loss in a first spectral sub-region of the emitted light can thus be greater than in a second sub-region. The organic layer stack of an OLED can be viewed as a microcavity, in which a layer generating organic light is embedded, in which light emission takes place due to luminescence when an external voltage is applied. The geometrical boundary conditions in the microcavity cause certain sub-regions of the emitted spectrum to be suppressed or even completely cut off, so that effectively other sub-regions of the spectrum are emphasized in the radiated light. This can result in an undesirable reduction in the Color Rendering Index (CRI).
Take the influence of the distance of the organic light-emitting layer from a reflectively formed electrode surface as an example. In the event of a change in the distance due to increasing or decreasing the layer thickness of the layers arranged in between, the position and width of the spectral sub-regions that are suppressed or enhanced in the radiated light change, so that a different radiation characteristic of the component is yielded.
To enhance the Color Rendering Index, measures are known, for example, to adjust and optimize the spectrum of the radiated light by suitable positioning of the light-emitting layer in the microcavity. Furthermore, the addition of additional light-emitting layers, which ensure additional emission in individual, limited wavelength ranges, can enhance the Color Rendering Index. The manufacture is particularly elaborate, however, and can only be realized by the use of a cluster device. As well as this, such a procedure is accompanied by a rise in the required operating voltage.